The present invention relates to a power transmission that transmits the driving force of an engine comprising a plurality of cylinders, to wheels through a drive-power transmission (for example, a transmission). Furthermore, the present invention relates particularly to a power transmission which comprises a frictionally engaging element (for example, a starting clutch) that variably sets the capacity of the drive-power transmission to transmit the driving force.
Many automobiles are equipped with such power transmissions. Recently, for the purpose of improving fuel economy, vehicles are equipped with an idling elimination control, in which the operation of the engine is stopped when the vehicle comes into a halt, or with a partial cylinder operation control, in which, under certain driving conditions, some of the cylinders are disengaged from the operation of the engine. For example, Japanese Laid-Open Patent Publication No. S59(1984)-13154 discloses a control for increasing the speed change ratio of the ratio-change mechanism used with such an engine, which is equipped with cylinders that are disengageable when the engine is in a partial cylinder operation mode.
In comparison with an all cylinder operation mode, where all the cylinders are used for the operation of the engine, the output of the engine in a partial cylinder operation mode is smaller because some of the cylinders are disengaged from the operation. If the power transmission, which transmits the output of the engine to wheels, is controlled in the same way for the engine operated in a partial cylinder operation mode and for the engine operated in an all cylinder operation mode, then there are problems. For instance, the load for the engine in a partial cylinder operation mode may become too heavy, and as a result, it may cause an engine stall. Also, it may impair the acceleration performance of the vehicle. Furthermore, the operation of the engine in a partial cylinder operation mode can occur not only for fuel saving, where the operation of the engine with some cylinders only is intentional, but also accidentally if the controller of the engine operation fails or breaks down and prevents some cylinders from participating in the operation of the engine.
To solve the above mentioned problems, it is an object of the present invention to provide a power transmission which has a predetermined acceleration performance and which will not cause any engine stall in a partial cylinder operation mode.
To achieve these objectives, the present invention provides a power transmission that comprises an engine, a drive-power transmission (for example, the continuously variable transmission CVT described in the following embodiment), a frictionally engaging element (for example, the starting clutch 5, the forward clutch 25 and the reverse brake 27 described in the following embodiment) and a transmission-capacity controller (for example, the control valve CV described in the following embodiment). The engine has a plurality of cylinders, and the drive-power transmission transmits a rotational driving force from the engine. The frictionally engaging element can set variably a transmission capacity for the drive-power transmission, and the transmission-capacity controller controls the engagement operation of the frictionally engaging element. While the engine is in a partial cylinder operation mode, where some of the cylinders are disengaged from the operation of the engine, the transmission-capacity controller controls the engagement operation of the frictionally engaging element to attain a predetermined transmission capacity at a rotational speed of the engine that is higher than for a case of the engine in an all cylinder operation mode, where no cylinder is disengaged from the operation of the engine.
With the power transmission constructed as described above, while the engine is in a partial cylinder operation mode, if the driver tries to accelerate the vehicle, the engagement operation of the frictionally engaging element is controlled to attain a predetermined transmission capacity at a rotational speed of the engine that is higher than for a case of the engine in an all cylinder operation mode. If the output of the engine in the partial cylinder operation mode becomes smaller, then the rotational speed of the engine is increased so as to increase the output, and then a power transmission through the drive-power transmission is executed. As a result, engine stall is effectively prevented, and a desired acceleration performance is secured for the vehicle.
Preferably, the drive-power transmission comprises a ratio-change mechanism, which transmits the rotational driving force of the engine with a rotational speed change, and the frictionally engaging element comprises a starting clutch, which is provided in the ratio-change mechanism, so that the rotational driving force is transmitted through the ratio-change mechanism with a rotational speed change to wheels to drive the vehicle. With this arrangement for the power transmission, while the wheels of the vehicle are stationary, when the ratio-change mechanism is set into a drive range, the transmission-capacity controller controls the engagement operation of the frictionally engaging element to transmit a creep torque through the ratio-change mechanism to the wheels if the engine is in an all cylinder operation mode but disconnects the frictionally engaging element to make a driving torque transmitted to the wheels almost nil if the engine is in a partial cylinder operation mode. In this way, while the vehicle is stationary in an in-gear condition (i.e., a condition where the transmission is set in a drive mode), the frictionally engaging element can be controlled to transmit a creep torque if the engine is in an all cylinder operation mode or to transmit no creep torque if the engine is in a partial cylinder operation mode. As a result, engine stall is effectively prevented.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.